1. Field of the Invention
This invention relates generally to an optical angle-of-arrival measurement system, and more particularly to an optical angle-of-arrival (OAOA) measurement system and method capable of measuring the OAOA for light arriving from multiple sources simultaneously.
2. Description of the Related Art
High precision and large field-of-view (FOV) optical angle-of-arrival (OAOA) measurement is a critical task in adaptive optics, laser communications, target tracking, optical surveying, and many other applications. Current solutions are limited either by resolution or FOV due to inadequate focal plane array (FPA) size and detector noise. Conventionally, a lens converges incoming light from a fixed location point source onto an FPA that has M (row)×N (column) pixels. The focused spot on the FPA is circular (e.g., having a Gaussian intensity distribution). The position (x, y) of the focused spot on the FPA is determined, from which the optical angle of arrival is calculated as θx∝x/f, θy∝y/f, where f is the focal length of the lens. The resolution (δθ) of the OAOA measurement will be proportional to d/f, where d is the FPA pixel width. Improving resolution requires reducing pixel size (d) and/or increasing focal length (f), both of which reduce the FOV.
FOV can be improved with an FPA having a small pixel size and a large number of pixels, but such an FPA is expensive to fabricate. Furthermore, a large number of pixels will slow down the frame rate of the sensor and increase the processing time required to determine the spot location.
One approach to improving the accuracy with which the location of a focused spot on the FPA can be determined is described in co-pending U.S. patent application Ser. No. 12/401,027. Here, optical elements located in front of an FPA convert incoming light into two perpendicular narrow width lines which are oblique with respect to the FPA's row and column axes and which form a crosshair pattern on the FPA. A means for interpolating the position of the lines on the FPA is used to provide x, y coordinates of the point at which the lines intersect, which can be used to calculate the optical angle of arrival in accordance with θx=A(x)·tan−1(x/f), and θy=B(y)·tan−1(y/f), where f is the focal length of the optical element, and A(x) and B(y) are parameters that account for optical distortion and other imperfections of the system.
However, a problem can arise if light originating from multiple sources is simultaneously received and focused on the FPA, which results in multiple crosshair patterns being formed on the FPA. When this happens, it may be difficult or impossible for the interpolation means to determine the correct intersection point for each crosshair pattern, especially when the crosshairs overlap.